(Not applicable)
(1) Field of the Invention
The present invention relates generally to drywall construction, and more particularly to an improved corner bead strip with paper wings.
(2) Background Information
Current building construction utilizes sheets of drywall, commonly referred to as xe2x80x9cwallboard,xe2x80x9d to form the surfaces of interior walls of buildings. Drywall, or wallboard, is typically formed of sheets of plaster sheathed in an outer wrapping of heavy construction paper.
In wallboard construction, joints between adjacent sheets of drywall are usually covered by a paper tape extending lengthwise along the joint. The conventional drywall tape is provided in narrow elongated strips of porous paper wound into rolls. The drywall tape is applied to the joints, and then covered with wet plaster or xe2x80x9cmudxe2x80x9d. The plaster is feathered and smoothed along the edges of the tape to conceal the tape edges and form a smooth unmarred surface where the wallboard adjoins.
It is often necessary to cut the wallboard to form a corner, which thereby exposes the plaster contained between the heavy paper sheets. This exposed plaster tends to crumble unless these edges are protected. To finish exterior corners in wallboard construction, metal corner beads and bullnose beads are typically installed. Such corner beads are conventionally formed by roll-forming from an elongated strip of sheet metal, and provide a round nose with two mounting flanges extending at substantially right angles from the opposing sides of the nose. These mounting flanges are often knurled or embossed to provide a rough surface so that the joint compound will adhere when the corner is finished. Similarly, metal trim is used to protect and finish a wallboard edge at window and door jams, while expansion joints are inserted between sheets of wallboard at predetermined intervals within buildings.
The corner bead is installed by securing the mounting flanges along the surface of the drywall panels adjacent to the corner by nails or the like. Wet plaster is then smoothed into place to cover the metal flanges, and edges of the plaster are smoothed and feathered to cover and conceal the metal edges.
Another type of corner bead is referred to as a xe2x80x9ctape-onxe2x80x9d bead. Tape-on corner beads utilize a strip of paper covering the exterior surface of the metal corner angle, with wings projecting outwardly from the legs of the corner angle. Wet plaster or joint cement secures these paper wings to the drywall, to secure the corner bead in position, rather than using nails or other fasteners. Wet plaster or joint cement for finishing the corner will normally adhere significantly better to the paper cover strip of tape-on beads, than to the exposed metal of conventional nail-on beads. Nail-on beads are also typically more susceptible to developing crack lines along the outer edges of the flanges, than are tape-on beads. In addition, tape-on beads are more tolerant of dimensional and geometric changes in the underlying construction framing than are nail-on beads with their rigid mechanical attachment to the construction framing.
One of the main problems with prior art tape-on bead is the use of standard joint/drywall tape on the bead. Such drywall tape is very fibrous, which is good for bond strength, but poor for appearance. During the application of joint cement over the tape, to adhere the corner bead to the drywall, fibers will project and protrude with only minimal contact by the application tools. These fibers will ball up during the course of sanding of the joint cement for the final finish, thereby detracting from the finished appearance of the corner.
One method for improving protection against adverse abrasion of this paper strip is disclosed in U.S. Pat. Nos. 5,613,335 and 5,836,122, both to Rennich et al. These patents disclose a paper bead (tape-on bead) utilizing a paper layer which is uniformly impregnated throughout its thickness with latex or similar strengthening compound with a high wet strength so as to make the paper strip resistant to scuffing and abrasion throughout its thickness. This impregnated stock paper would have a high pick resistance or surface fiber bond, and would effectively inhibit separation of surface fibers during application on wallboard, thereby providing a good finished appearance in installation. However, the applicants herein have found that paper of this type, which is impregnated with latex or the like, exhibits poor joint compound bonding properties, as applied under the ASTM bond strength tests.
More specifically, ASTM C-475-01 recites a variety of standards for joint tape, including the following three standards:
4.3.3 Tensile Strengthxe2x80x94Shall be not less than 30 lbf/in. (524 N/mm) in the cross direction.
4.3.4 Dimensional Stabilityxe2x80x94Shall expand not more than 0.4% lengthwise and not more than 2.5% crosswise.
4.4.1 Bondxe2x80x94between the joint tape and joint compound shall be not less than 90%.
These terms are used throughout this application, and are intended to refer to the standards set forth in ASTM C-475-01. The test method for determining bond strength or the xe2x80x9cpeel bondxe2x80x9d percentage recited in ASTM C-475-01, is set forth in ASTM C-474-01. Section 3.2.1 defines xe2x80x9cbondxe2x80x9d as the xe2x80x9cquality of adhesion between the paper joint tape and joint compound. Thus, the inventors herein have also described this as the xe2x80x9cpeel bondxe2x80x9d, or the amount of force required to peel the joint tape from the joint compound.
Section 13 of ASTM C-474-01 describes the methodology for calculating the percent bond set forth in ASTM C-475-01. The pertinent portions of that section are set forth herein in more detail, for a better understanding of the bond percent. First, the apparatus used in the sampling are described, and include xe2x80x9cfeeler gage stripsxe2x80x9d, and an xe2x80x9coverlay transparency gridxe2x80x9d. The feeler gage strips are described in 13.4.1 as being xe2x80x9c12 in. (300 mm) long, xc2xd in. (13 mm) wide, and 0.025 in. (0.64 mm) thick with a small hole drilled in one end. The overlay transparency grid is defined in 13.4.4 as xe2x80x9ca transparent photocopy of 10 by 10 divisions/in. graph paper. An area 2 by 5 in. (51 by 127 mm) enclosing 1000 square divisions is outlined.
The Procedure for determining percent bond is described in Section 13.5 as follows:
13.5.1 Place two feeler gage strips parallel to each other about 4 inches (102 mm) apart and fasten to the face of the gypsum wallboard with a thumb tack through the hole in the end.
13.5.2 Using the steel reinforced broad knife, apply an amount of joint compound sufficient to cover the area between the feeler gage strips. Spread the specimen evenly between the feeler age strips leaving the specimen slightly thicker than the strips.
13.5.3 Center a 12-in. (305 mm) length of paper tape in the specimen. Press one end of the tape into the specimen and hold it in place.
13.5.4 Embed the tape by applying two or three pressure strokes with the steel-reinforced broad knife. Wipe away from the end being held so the excess joint compound is squeezed out.
NOTE 8xe2x80x94The thickness of the joint compound plus the tape is about 0.025 in. (0.64 mm).
13.5.4.1 Carefully remove the feeler gages before drying.
13.5.5 Allow the test assembly to dry to constant weight in an atmosphere of 75+/xe2x88x925xc2x0 F. (24+/xe2x88x922xc2x0 C.) and 50+/xe2x88x925% relative humidity.
13.5.6 When the test assembly is dry, use a sharp knife to make a cut across and perpendicular to the tape 3xc2xd in. (90 mm) from one end. Make a second cut 5 in. (130 mm) from and parallel to the first cut. Make two diagonal cuts across the tape connecting the opposite corners of the 5-in. (130 mm) section. With the tip of the knife, peel back the tabs formed by the xe2x80x9cXxe2x80x9d cuts and pull up sharply.
13.5.6.1 Make a second test by repeating 13.5.6 below the first test.
13.5.7 Using a sharp pencil, lightly outline the area where fiber remains attached to the compound. Align the overlay transparency grid so that the grid outline matches the 2 by 5-in. (51 by 127-mm) sides of the tape bond area.
Section 13.6 of ASTM C-474-01 then sets forth the procedure for calculating the percent bond, as follows:
13.6.1 Using the overlay transparency grid, count the number of squares that are more than half bare of fiber separated from the tape and outlined by pencil.
13.6.2 Subtract this number from 1000 and divide by 10 to determine the percent bond. Record the average of the two tests.
As described above, xe2x80x9cpick resistancexe2x80x9d relates to the surface bond of the joint tape, and the resistance to balling of fibers upon finishing or dressing a corner by sanding. The paper exhibits high pick resistance if there is little or no balling or protruding of fibers upon dressing the corner. On the other hand, the paper exhibits low pick resistance if there are protruding fibers or balling upon the dressing of the corner.
It is therefore a general object of the present invention to provide an improved tape-on corner bead with paper wings, which exhibits high bond strength.
Yet another object is to provide an improved tape-on corner bead which will firmly bond to the drywall construction, the supporting metal corner angle, as well as the joint cement applied over the top thereof.
A further object of the present invention is to provide a method for constructing tape-on corner bead which permits secure attachment of the corner bead to wallboard.
These and other objects will be apparent to those skilled in the art.
The corner bead of the present invention is of the tape-on type, having an elongated metal core strip with a longitudinal arcuate nose and a pair of flanges extending outwardly from the nose. A cover strip of paper is bonded to the exterior surface of the core strip, and includes wings that project outwardly beyond the extent of the flanges. The cover strip is formed of a non-latex-bearing stock paper having high pick resistance and which is dimensionally stable and has high peel bond strength when in contact with wet joint compound.